1. Technical Field
The present invention relates to an asynchronous transfer mode (hereinafter, referred to as "ATM") switch system, and more particularly, to an improved assignment method and apparatus of a virtual path and virtual channel identifier information in the ATM.
2. Background Art
Generally, ATM is a specific packet-oriented transfer mode using an asynchronous time division multiplexing technique where both line switching and packet switching are unified and many pieces of information such as voice, data, and pictures are organized in fixed-sized blocks, called cells. Thus, ATM can operate as both a packet exchange system and a circuit exchange system in one transmission mode for efficiently embodying both high speed and wide band communication networks. For this reason, ATM is a target transfer mode solution for implementing a broadband integrated services digital network (B-ISDN) in comparison with a conventional synchronous transfer mode (STM) which can only transmit subscribers' information on channels having a fixed band.
In principle, conventional STM is a time position multiplexing scheme in which a channel is identified from a time slot within a frame. In the STM, the time slot occurs periodically regardless of the existence of information transmission. ATM, on the other hand, does not require the allocation of time slots on each terminal. Both the ATM and the STM are similar in the sense that they transmit information in a digital format and provide several channels in a transfer stream for support. There is however, one significant difference between the two systems regarding the method for assigning and identifying information channels. That is, the STM assigns information to be transferred through a fixed channel and identification of the assignment is performed according to relative location of a frame pattern. Thus, in a STM, the band channels are not effectively used because the band channels are continuously assigned even where no information is being transferred. In an ATM, on the other hand, the band channels are assigned only where information to be transmitted occurs; the band channels that are not assigned can be used for information transmission by another subscriber. As a result, ATM offers a more effective channel management. Information channel identification in the ATM is embodied by the virtual path and virtual channel identifiers (hereinafter, referred to as "VPI & VCI") or similar connection identifiers which are placed in a header of each cell for identifying cells belonging to the same virtual channel on an asynchronous time division multiplex. Such channel identification of the ATM facilitates the use of a multiplex, de-multiplex and exchange of digital information, and serves to decrease the costs of broad-band communication network elements as well as providing flexibility in a bandwidth distribution.
In a communication system or switching system using the ATM, a call processing unit placed inside the communication system or the switching system assigns the VPI and VCI values to each ATM cell whenever the subscriber requires a call connection. The assignment of VPI and VCI values is accomplished by reading in order of VPI and VCI values which are not under assignment every time the subscriber requires a call connection, and thereafter, the VPI and VCI values (or numbers), which are predetermined, are stored in a non-volatile memory. A call which receives the assignment of the VPI and VCI placed in a header area of an ATM cell is transferred through a transferring channel. A receiving party analyzes with a header conversion table memory the VPI and VCI values received, then processes a route for the call from a calling party. Header conversion table memory stores the VPI and VCI values for identifying a call and is used in conjunction with a self-routing switch for enabling a call connection between two terminal units to communicate with each other through the ATM switching system by virtue of the combination of VPI and VCI values included in the header area of an ATM cell. The problem is that if the VPI and VCI values for identifying a call are used as addresses for a header conversion table memory, the capacity of the header conversion table memory becomes considerably larger which consequently attributes to delay of call processing speed by the ATM switching system.
For instance, if the length of a VPI field of a cell header is 12 bits and the length of a VCI field is 16 bits used in conjunction with a Network Node Interface (NNI) as recommended by CCITT recommendation I.361, a header conversion table memory in the ATM switching system would require an address capacity of 2.sup.28 =256 Mbits. Even in a context of a User Node Interface (UNI) where the length of a VPI field of a cell header is only 8 bits and the length of a VCI field is 16 bits, a header conversion table memory would still require an address capacity of 2.sup.24 =16 Mbits. Since the total memory capacity of the header conversion table memory is considerably large, in the order of G bits, the switching operation of the ATM switching system slows down considerably.
One improvement on the conventional VPI and VCI assignment technique is disclosed, for example, in U.S. Pat. No. 5,119,369 for "Packet Switch Communication Network Using Packet Having Virtual Channel Identifier" issued to Tanabe et alii, which envisions to reduce the capacity of the header conversion table memory by using an asymmetrical VCI having a value which differs depending on the direction of transmission of the information ATM cell between two terminal units which communicate with each other, thereby enabling identification of a logical connection between the two terminal units using only the VCI values. As a result, the address capacity of a header conversion table memory for rewriting a header label of a received ATM cell can be limited to the length of a VCI field which is only 2.sup.16 =64 Kbits instead of 2.sup.28 =256 Mbits or even 2.sup.24 =16 Mbits. The sequential assignment of the VPI and VCI values in the header area of the ATM cell still however prolongs a call processing speed of a call processing unit since the call processing unit has to search for the VPI and VCI values under a "free status" (i.e., a current unused condition or release condition) whenever little call connection is required. In the initial operation for assigning the VPI and VCI values in a cell header, there is a substantial problem associated with the sequential assignment of the VPI and VCI values in response to a connection requirement specified by a call. After all the VPI and VCI values are assigned from the beginning to the end of the relevant range however, the call processing unit must hunt for a VPI and VCI value that is under a "free status." For example, if the relevant range of VPI and VCI values within a cell header is 24 bits in conjunction with a User Network Interface (UNI), or 28 bits in conjunction with a Network Node Interface (NNI), the call processing unit must check up to a total of 2.sup.24 =16 Mbits or 2.sup.28 =256 Mbits address locations in the header conversion table in order to seek for the VPI and VCI values under a "free status." Consequently, a call processing speed of the call processing unit suffers as a result of such checking operation. As described above, the sequential assignment of VPI and VCI values in the conventional ATM switching system slows down the call processing speed of a call processing unit which negates any recent improvements made on the call processing speed.
There are other techniques for improving the switching ability of a data switching system such as, for example, in U.S. Pat. No. 4,890,280 for "Frame Relay Type Data Switching Apparatus" issued to Hirata, which endeavors to reduce the number of times of memory access and allows header updating to be executed with no regard to the capacity of a call processing unit, thereby enhancing the switching ability. In Hirata '280, the data switching system separates header data from an incoming communication channel and compares the separated header data with incoming channel numbers while communication data is temporarily stored in a first-in-first-out memory in order to subsequently combine outgoing channel numbers with stored communication data to provide channel data for an outgoing communication channel. In order to separate header data from the incoming communication channel, a call processing unit would still search for VPI and VCI values under a "free status," which would inevitably prolong a call processing speed. Thus, the data switching system as disclosed by Hirata '280 still suffers the same deficiency as that of Tanabe et al. '369.